Milley, a four-star general, is being nominated to succeed current Gen. Joseph Dunford, whose second term as the nation's highest-ranking uniformed officer will end in the summer of 2019.

"I am pleased to announce my nomination of four-star General Mark Milley, Chief of Staff of the United States Army – as the Chairman of the Joint Chiefs of Staff, replacing General Joe Dunford, who will be retiring," Trump tweeted Saturday morning.

"I am thankful to both of these incredible men for their service to our Country! Date of transition to be determined," he added.

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Trump had teased the announcement on Friday on the eve of the annual Army-Navy football game in Philadelphia, which the president is slated to attend later Saturday.

Former President Obama nominated Dunford and current Vice Chairman Gen. Paul Selva in 2015, and Trump last year renewed them for second terms.

If confirmed by the Senate, Milley would serve as Trump's top military adviser, providing input on U.S. strategy in Iraq and Syria as well as Afghanistan, where U.S. forces have been deployed for 17 years.

Milley, a Massachusetts native and Princeton University graduate, served three tours in Afghanistan for Operation Enduring Freedom and served as deputy commanding general of U.S. forces in Afghanistan.

He's also served in Iraq, Panama, Haiti, Bosnia-Herzegovina and Somalia, among other countries.

News of his appointment comes amid a broader shake-up of Trump's Cabinet and senior staff.

Trump announced Friday that he would appoint William Barr as his next attorney general and State Department spokeswoman Heather Nauert as the new U.S. ambassador to the United Nations.